Transmitter for radio seismic systems



Feb. 2, 1954 C. MONTGOMERY TRANSMITTER FOR RADIO SEISMIC SYSTEMS Filed Sept. 20, 1948 2 sheets-sheet 1 A TTORNE YS Feb 2, 1954 o. c. MONTGOMERY 2,668,282

TRANSMITTER FOR RADIO SEISMIC SYSTEMS Filed Sept. 20, 1948 2 Sheets-Sheet 2 INVENTOR. ORlN C. MONTGOMERY BY e ATTORNEYS Patented Feb. 2, 1954 TRANSMITTER FOR RADIO SEISMIC SYSTEMS Orin C. Montgomery, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application September 20, 1948, Serial No. 50,202

3 Claims. (Cl. 340-15) This invention relates to a radio transmitter particularly adapted for providing an indication of shot time in a radio seismic system. In another aspect, it relates to a receiver for recording the shot time at a location remote from the transmitter. In still another aspect, it relates to a method of transmitting seismic time breaks. In still another aspect, it relates to a method of and apparatus for simultaneously transmitting time break signals and other intelligence over a single radio channel.

In seismic systems, an explosive charge is detonated in the earth at a location identified as the shot point. The explosion produces seismic Waves which are reflected from subterranean strata and received at several locations by seismometers or geophones, which convert the received seismic waves into electrical voltages representative thereof. These voltages are ordinarily recorded by a common unit, the time elapsing between the detonation and the arrival of the seismic waves at the respective geophones yielding valuable information regarding the subterranean strata.

From the foregoing, it will be apparent that the detonation time, commonly referred to as the time break or shot time, must be recorded with a high degree of accuracy to permit useful measurements to be made of the period between the time break and the time at which the seismic waves are received by the seismometers. It has been proposed to transmit a radio signal at the instant the time break occurs, and utilizesuch signal to actuate a receiver at the recording location, the output of the receiver being fed to one of the galvanometer units of the recorder to provide a time break indication.

However, considerable diiculty has been experienced in providing avradio linkage for transmitting signals which indicate shot time in a seismic system to a receiver which is remote therefrom. In such a system, it is desirable that the same channel over which the time break signals are transmitted may be also used for carrying speech or other intelligence. In the methods and apparatus of the prior art, this has necessitated elaborate switching arrangements which have not proved reliable on a practical and commercial scale.

It is an object of this invention to provide a novel radio transmitter and receiver for transmitting time break signals with or without the simultaneous transmission of speech or other intelligence.

It is a further object of the invention to provide a receiver for separating the speech and time break components in the transmitted radio waves.

It is a still further object of the invention to provide an improved method of and apparatus for transmitting and recording the time break information.

It is a still further object of the invention to provide apparatus which is reliable in operation, rugged in construction, low in cost, and which may be readily manufactured from standard components.

Various other objects, advantages and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a block diagram of the transmitter;

Figure 2 is a block diagram of the receiver; and

Figure 3 is a schematic diagram of certain portions of the circuit illustrated by Figure 1.

Referring now to the drawings in detail, and particularly to Figure 1, the transmitter comprises an audio frequency oscillator I Il whose frequency of oscillation is controlled by a phase shift network so as to normally produce audio frequency oscillations of a predetermined frequency. A frequency modulator II Works into the oscillator and, in general, it is connected in the phase shift network in such fashion that detonation of an electric blasting cap included in the frequency modulator circuit produces an abrupt shift in frequency of the oscillator Ill. The audio frequency signal from oscillator I0 is fed to an amplifier I2 and the output of this amplifier is used to amplitude modulate a radio frequency carrier wave produced by a transmitter I3. Thus, in effect, the oscillator It produces a subcarrier which amplitude modulates transmitter I3 and which is, in turn, frequency modulated by the unit I I. In accordance with the invention, transmitter I3 is also amplitude modulated by the output of an audio or speech amplifier I II which is fed by a microphone I5. The speech signals from microphone I 5 and amplifier I 4 do not cause any appreciable interference with the signals from frequency modulated audio oscillator I 0 since the latter signals consist principally of low frequency amplitude changes, with a very small or negligible frequency variation at the frequency of oscillator Ill'.

The signals from the transmitter of Figure 1 are received by the apparatus of Figure 2 which includes a conventional amplitude modulation receiver I5 for demodulating the carrier wave, thereby providing an audio frequency voltage representative of the modulating signals fed to the transmitter I3. The receiver has two channels, one for demodulating and recording the frequency modulated signal produced by detonation of the blasting cap, and the other for reproducing the speech or other intelligence fed into the microphone l of Figure 1. The first channel {com-prises an amplifier 'lil which'is tuned to the narrow band of frequencies produced by the oscillator l0 and modulator Il of Figure l, this band of frequencies being in the neighborhood of 3,000 cycles per second. {The-.output of amplier l1 is fed to a limiter I3 which operates, in a known manner, to eliminate the portions of the signal which are of greater fthan -a predetermined magnitude. Thus, the output of `the limiter consists of a series of square waves of constant amplitude but varying frequency which are fed to discriminator I9.

The unit i9 is also of conventional construction and its function is to demodulate or detect the frequency Avariations :in :the incoming signal. Discriminator i9 utilizes a balanced circuit so that, vdur-ing Ioperation of oscillator t0 at 'fits normal predetermined frequency, no output wol-tage is produced. This occurs when the `oscillator is operating at said normal frequency before the de- -tonation of the blasting cap. When the blasting cap is broken by detonation of an explosive Acharge, that lis, when :a time lbreak occurs, fre- Iquency modulator 'H causes the frequency of the oscillator signal to deviate from its normal Vfrequency. Thereupon, discriminator i9 produces an output 4vol-tage of generally sinusoidal wave for-m whose amplitude is proportional to the frequency deviation and this voltage is fed to lan raudio amplifier 213 4which dri-ves a galvanorneter "24 forming a part -of a suitable recorder uni-t. In `-this manner, the shot time or time break is accurately indicated by the voltage pulse impressed upon the galvanometer. Since a frequency modulation circuit is used, transient voltages have Very little effect upon the system and, hence, do `not affect the reading of the galvanometer. lt 'has been found that the system functions with very lhigh iidelity to provide an indication of `the exa-ct instant at which the time break occurs.

The second Achannel of the receiver `includes a filter 22 `driving an audi-o amplifier 23, the output of which is fed to a speaker 26. The use of filter 22 is preferred but it is not essential to the operation of the invention, its function being to eliminato signals within the band utilized by the frequency modulated oscillator 10. lSuch filters are well `known in the art and, hence, are not described in detail herein.

' It ywill be apparent that the transmitter and receiver-of this invention may be utilized for the simultaneous `transmission of time break information and of speech or other intelligence. the frequency* Ymodulated signals are excluded from audio amplifier 23 and speaker 24 by the filter 22 while the speech voltages are Vsubstantially completely excluded from the frequency modulation portion of the receiver by tuned amplier H. The strength of the frequency modulated signal should be suicient that the portion of the speech frequency band passed by tuned amplifier II is not of sufcient strength as 'to operate the discriminator Vand -produce an indication on galvanometer 2i 'The band width utilized for the frequency modulation signal is so narrow that this can vvbe accomplished without diiiiculty and .elimination of this frequency band by the filter 22 does not appreciably lessen the intelligibility of the speech transmitted through audio amplifier 23 and speaker 24. Accordingly, the present system permits a simultaneous transmission of speech and time break impulses without the necessity for manipulating switches or making other adjustments. The use of the frequency modulation system for transmitting the time break impulse prevents interference with `the signal by `outside noises, such as static, `by microphone noises, or by noise voltages generated in the transmitter circuit. Also, the frequency modula- :tion icircuits )permit transformers and resistorcondenser circuits tending to introduce a time delay to be substantially eliminated.

"The'phaseshift oscillator IG, frequency modulator 1| and ampliiier I2 are shown in detail by the schematic circuit diagram of Figure 3. The oscillator icircuit includes a tube 26 having an anode, .a cathode, and a control grid. In the present example, the tube 2,6 is a pentode and has, in addition, a screen grid and suppressor grid but these extra elements are not essential lto the functioning yof the linien-tion. 'The suppressor `grid ci the tube isconnected to the cathode which :is grounded through a resistor '-21 shunted by a condenser "28. An operating `potential is supplied to the anode from apcsitive power supply terminal 2'9 through resistors 3B and 3l, the screen grid "being connected vtoV terminal 2li through a -resi-stor I32 vand to ground through -a filter condenser 33.

A phase shift network, generally indicated 'by reference character is connected fin the alf-iode lcircuit of oscillator tube `226, this network includ- Ving three series connected -condensers 36, 3'! and 38 with grounded resistors '39 Aand 140 lconnected to the junctions between said condensers. 'The network also includes a resistance unit 1li! Iconnected between 'its output lead nand ground, 4the output of the network being fed through a variable coupling condenser l2 to the `control grid of a cathode follower tube I3. 'The grid -of tube 43 has a Igrid resistor M connected to the cathode thereof and the cathode 'is rgrounded through a load resistance 45. The condenser K42 and resistor 44 also vform a `part o-f the phase shift networ-k, and condenser -42 is variable to adjust Ethe frequency ofthe phase shirt oscillator, as will be apparent from the following description. The anode of tube 43 is connected -to a pesi-tive termi nal 4S ofthe power supply. The output of the tube 43 appears between the cathode and ground and this output is fed back to the grid of oscillator tube 26 by a lead y1H and `coupling condenser 48, the signal being impressed upon 4a grid resistor 49 connected between the grid of tube 2E and ground.

The network 35 is designed to provide Va 1180 Idegrec phase shift of the oscillator output voltage at a particular operating frequency determined by the values of condensers 31.5, 31, 38, and `42 and resistances 39, 40, M and 44. Thus, the network provides a degree phase shift between the anode and the control grid of tube '2%. As previously stated, this operating frequency is preferably within the audio range, for example, 3,000 cycles per second. The tube 43 functions as an impedance matching device between the control grid and anode circuits of tube 2t, increases the stability of the circuit, and also provides a requisite Yamount of gain needed to maintain tube 26 in an oscillatory condition.

Out put voltage is taken from the anode circuit of oscillator tube 26 by a lead 5I `connected to the junction between resistors 30 and 4 I, this lead extending through a coupling condenser 52 to one terminal of a potentiometer 53, the other terminal of which is grounded, and the movable arm of which is connected to the control grid of an amplifier tube 5d. The cathode of tube 54 is grounded through a resistor 5b which is shunted by a condenser 5E and operating potential is supplied to the anode of the tube from a positive power supply terminal 5l' through the primary winding of a transformer' 53. A secondary winding of transformer 58 is connected, in a known manner, in the plate circuit of the output tube of transmitter i3 to effect amplitude modulation thereof. Tube 5d functions not only as an ampliiler but also as an isolation circuit preventing interaction between oscillator is and audio plifier Hi.

When the blasting cap of the seismic system is broken by detonation of an explosive charge, the frequency of oscillator il is abruptly changed by varying the resistance of a portion of the phase shift network 35. Responsive to such a resistance change, the network 35 provides a 180 degree phase shift at a second frequency somewhat different from the original frequency of oscillation and, accordingly, the oscillator( signal is shifted to such second frequency. To this end, the resistance unit il is connected between output lead of the network and ground, this unit including series connected resistors E0, 6| and a condenser 62. Resistor 6i and condenser 62 are shunted by the plate resistance of a modulator tube 64 having its cathode connected to ground through a resistor E5 and having its anode connected through a lead 65 to the junction between resistors 653 and 65. is supplied to the anode from positive terminal 2S through a resistor 6! which is also connected to the junction between resistors S53 and 6l The control grid of modulator tube til is connected to the arm of a potentiometer S9 which is shunted by an electric blasting cap I0 connected in series with a battery 'l I.

Although the excitation circuit of tube 64 is shown as blasting cap 'iii and battery ll, by way of example, I preferably utilize a balanced f Wheatstone bridge circuit, as described in the copending application of Deslonde R. deBoisblanc and John E. Bondurant, Serial No. 628,310, filed November13, 1945, entitled Seismograph Blaster Time Breaker Circuit, now Patent No. 2,470,486, to apply a sudden sharp pulse of voltage to the grid of tube 64 when a time break occurs.

It will be apparent, therefore, that the plate resistance of tube 64 forms a part of phase shift network 35 and, with blasting cap 'lll connected in circuit, the potentiometer 59 is adjusted so that interruption of the grid circuit of tube 64 by detonation of blasting cap 70, produces a desired predetermined change in the plate resistance of tube 64. Condenser 52 is then adjusted so that tube 25 oscillates at its normal predetermined frequency. When the charge is detonated, the circuit of blasting cap lil is abruptly opened and the grid voltage of tube Sil is changed. As a result, there is a variation in the current Aflowing through the tube and a variation in its plate resistance. This changes the constants of phase shift network 35 and produces an abrupt change in the frequency of oscillator 2S, the amount of frequency deviation being controlled by the setting of resistor t9. At the receiver, this frequency deviation produces a signal in discriminator I9 and provides an audio voltage which is amplified and actuates recording gal- Operating potential n.. .ab

vanometer 2| thereby providing a record of the shot instant or time break.

The described circuit has been found to be very satisfactory because the frequency of oscillation may be very rapidly changed Without causing even a momentary interruption of the circuit oscillations. The circuit is very stable and is highly selective so that only a very narrow frequency band need be used to transmit the frequency modulated signals. The cathode follower 43 serves the purpose of matching the output impedance of the network to the input impedance of the tube 2t, provides gain between the network and grid of tube 26, and increases stability. In this connection, it will be understood that other types of networks may be used to provide the degree phase shift and, in some instances, other types of frequency modulated oscillators may be used. The circuits disclosed are very simple to adjust and may be used for simultaneous transmission of time break signals and speech without the necessity of manipulating switches or other controls.

While the invention has been described in connection with a present, preferred embodiment thereof, it is to be understood that this description is illustrative only and is not intended to limit the invention, the scope of which is defined by the appended claims.

Having described my invention, I claim:

l. Apparatus for transmitting time break signais and other intelligence over a single radio channel which comprises, in combination, a radio transmitter including an audio frequency oscillator, an electric blasting cap, a modulator circuit including said blasting cap and operated to vary the frequency of said oscillator responsive to the detonation of said blasting cap, means for producing a radio frequency carrier wave, means for amplitude modulating the carrier Wave with the output of said oscillator, means for further amplitude modulating the carrier wave with other intelligence, and a radio receiver including an amplitude modulation receiver for demodulating said carrier wave to produce an audio frequency output signal, first and second channels fed by the audio frequency output of said receiver, said first channel including a limiter and discriminator, an audio amplifier fed by said discriminator, and an indicator sensitive to the output of said audio amplier, said second channel including an audio amplifier, and a device responsive to said intelligence fed by the second channel audio amplifier.

2. Apparatus for simultaneously transmitting speech and time break signals over a single radio channel which comprises, in combination, a transmitter including an audio frequency oscillator having a phase shift network determining the frequency of oscillation thereof, an electric blasting cap, afrequency modulator circuit including said blasting cap, said circuit being operated to vary a component of said network responsive to detonation of said blasting cap, thereby to produce an abrupt change in the frequency of said oscillator, means for producing a radio frequency carrier wave, means for amplitude modulating said carrier wave with the output of said oscillator, a microphone, an audio amplier fed by said microphone, means for amplitude modulating said carrier wave with the output of said audio amplifier, and a radio receiver including an amplitude modulation receiver for demodulating said carrier wave to produce an audio frequency output signal, enr-st and 'second 'channels ied by the, audio frequency output of said receiver, said stir-'stehenial including an amplifier tuned tothe frequency Yoi? said foscii'laitoif, a limiter `and 'discriminator fed by *said tuned amplifier, an audio airiplifier 'fed by said discriminator, and 'a fgalvanometer sensitive to the 'output of said audio, ampli-fier, lsaid ysecond channel including an audio amplier, fand n speaker fed by the second 'channel audio amplier.

3. Apparatus for vsiniultaneousiy transmitting time break signals 'and speech signals over a single radio 'channel which comprises, in combination, a transmitter including 'an oscillator 'tube having an anode, a cathode and a control grid, fa phase :shift network Aconnected in the anode 'circuit of said tube Yconflprising three series connected 'cond-ensers, grounded resistors 'connected to the ijunction's between said condensers, and a third resista-nce unit connected between the outputcdnductor 'of said network and ground, a 'cathode `follower tube having an anode, a 'cathode, and a control grid. means for feeding the output of Said network to the 'control grid 'of said 'cathode follower, means 'connecting 'the cathode of said follower 'tube to the control lgrid of `said oscillator tube, 'a modulator tube having 'an anode, a cathode and a 'control grid, 'the anode-'cathode circuit of said modulator tube forming 'a part of said third resistance unit whereby va change in inteial resistance 'of the tube causes a `correspendi-ng Yshift -in the phase 'of the signal fed through said network, thereby to change the .frequency of said oscillator, 'an electric blasting cap, a 'detonating circuit including said blasting cap rconnected between the control grid of said modulator tub'e 'and ground whereby detonati'on 'of said blasting cap causes fa Change in the control grid voltage of Vsaid modulator tube there by changing the internal insistance thereof. means for supplying operating potentials to 'the anode and cathode 'of ea'ch tube, an amplifier, means 'for 'feeding `the output 'of vsaid oscillator tube to said amplier means for -feeding a speech signal to said -ampl'ien and means for amplitude modulating a radio frequency carrier with the output of said amplifier, and a receiver including an amplitude modulation receiver for demodulating ysaid 1carrier Wave to produce an audio frequency output signal, `first and Vsecond channels fed by the audio frequency output `of said receiver, said first channel including an amplifier tuned to the frequency of said oscillator, a limiter and discriminator fed by- `said tuned amplifier, an audio amplifier fed by said discrminator, and an indicating dev-'ice sensitive to 'the output 'of "said audio amplifier, said second 'channel including a filter .for eliminating vsignals of the frequency vof said 'f'oscillaton an 'audio ampliiier and a speechtransducing fdevice fed by the second channel audio amplien ORIN C. MONTGOMERY.

References Ci-ted in the iile of this patent NITED STATES PATENTS Number Name Date V1,950,406 Hoorn i Mar. 13, 1934 2,237,522 Clark v Apr. 8, 1941 '2,303,493 Purington Dec. l, 1942 2,407,308 Lorenzen et al sept. 1o, 1946 '2,413,116 SvvhOOkret al. V. Dec. '24., 1914.6 2,435,903 Rf/Zmn Fell 10, l1943 '2,457,137 Earp g Dec. 28 1'948 2,509,438 `Win`lerha1ter Sept. 2, 19,52

OTHER REFERENCES Radio Engineering, 3rd ed., by Terman, p. 437. 

